Cleaning is critical to most manufacturing processes. Solvents, which had long been considered the "ultimate" cleaners, are being largely eliminated from the arsenal of available cleaning tools in a world-wide environmental effort. Aqueous and semi-aqueous cleaners are the only viable options left for many applications. Ultrasonic excitation boosts the effectiveness of aqueous and semi-aqueous cleaners to exceed the quality and cost standards previously obtained by the use of solvents. Ultrasonic methods provide the ultimate in cleaning effectiveness and speed to satisfy the needs of the changing environmentally-sensitive manufacturing world.
Ultrasonic energy has the ability to reach inside partially closed areas such as part interiors, blind holes and crevices to give a mechanical boost to chemical cleaning where the use of a brush or other means is either impossible, ineffective, or time consuming. On a macro scale, this may include cleaning the interior or a transmission housing weighing several hundred pounds or on a micro scale, removing buffing compound residue from filigree work on expensive jewelry. The thoroughness of ultrasonic cleaning cannot be matched by any other method.
In ultrasonic cleaning, a solid state electronic generator converts standard electrical current into electrical energy of a higher frequency (typically 10-200 KHz). A transducer then converts this energy into mechanical waves. These transducers are either bonded to the exterior wall of a tank, or are enclosed in a stainless steel immersible housing which is mounted inside a tank. The sound waves produced by these transducers cause disruption of the liquid as alternative positive and negative pressure areas are produced resulting in vacuum cavities or cavitation bubbles. These bubbles are created during negative pressure periods, grow larger over several cycles and then collapse. The pressure exerted by the imploding bubbles accomplishes a scrubbing action which results in rapid, efficient and gentle cleaning. The small size of the bubbles permits their penetration into areas that cannot be reached using brushes or sprays.
There are several problems associated with manufacturing an effective ultrasonic cleaning apparatus. In some applications, the cleaning fluid is corrosive. This requires that the ultrasonic cleaning tank be made of a compatible corrosion-resistant material, such as stainless steel, quartz or a more exotic material for certain acids. It also is imperative that the transducer be properly coupled to the liquid so that the ultrasonic energy is effectively transferred from the transducer to the liquid in the tank. A preferred method of attachment of the transducer element to the exterior wall of the tank or to the immersible housing is vacuum brazing. Since vacuum brazing is best accomplished between two similar metals, transducers have, in the past, been secured to a stainless steel brazing mass (by epoxy, for example) and the brazing mass was brazed to the wall of the tank. A preferred method is that of vacuum brazing.
Another problem encountered by manufacturers of ultrasonic cleaning equipment is that of cavitation erosion. Cavitation at the liquid-solid surface boundary has been the subject of many articles. The two mechanisms thought to be responsible near the surface are microjet impact and shock wave damage. At the interface boundary, deformation of the collapsing cavitation bubble induces a fast-moving stream of liquid toward the surface with velocities greater than 100 meters/second. Surface pitting is the result of these microscopic impacts. Shock waves created by the collapsing cavity are also produced. One estimate of the peak pressures created is 500 atmospheres, which is half the pressure at the deepest region of the ocean, the Mariana Trench. Both mechanisms are known to exist, but the relative importance of each is a matter of debate. These mechanisms are responsible for cleaning. The effects of microjet streaming and shock waves expose, by breaking through the surface boundary layer, the base surface of the materials being cleaned.
The materials of which the ultrasonic tank are made are attacked at the point of maximum vibration by these same mechanisms over long hours of operation. To prevent cavitation damage, surface coatings such as hard chrome and titanium nitride have been used in the industry for many years. These materials reduce cavitation erosion which is considered to be a mechanical mechanism, by increasing the surface hardness. A 2 mil hard chrome coating has a Rockwell C hardness of 60, as compared to 25 for 316L stainless steel. Endurance testing has shown a reduction in surface cavitation erosion by a factor of 10.
In certain industries, the release of certain metals into the cleaning media due to even very mild cavitation erosion is very harmful. For instance, chromium will attack the silicon substrate used to manufacture semiconductors.
A new cobalt-base alloy has demonstrated resistance to cavitation erosion and corrosion. This alloy, sold under the trademark ULTIMET.RTM. by Haynes International, Inc. of Kokomo, Ind., demonstrates high elastic resilience, high yield strength and phase transformations. The alloy also demonstrates high resistance to cyclic fatigue. Surprisingly, despite the known features of this new alloy, no one has as yet used this alloy in a housing of an ultrasonic cleaning apparatus. Perhaps one reason for this is the high cost of the alloy. Perhaps another reason is that no one has heretofore discovered how to vacuum braze a stainless steel brazing element to the cobalt-base alloy wall (since it is cost prohibitive to construct the brazing disk from cobalt-base alloy as well). Unfortunately, manufacturing an ultrasonic cleaning apparatus having a tank constructed of one material and brazing member constructed of another dissimilar material creates problems that must be solved Copper or other metallic vacuum brazing requires that the parts to be brazed be slowly heated in a vacuum chamber to 2000.degree. F. at which point the copper melts and surface tension holds the parts closely together. With dissimilar materials being brazed, one of the materials will have expanded more or less than the other. As the parts are cooled, the copper solidifies joining the parts together but as additional cooling occurs the parts are under considerable stress due to the difference in thermal expansion of the parts. This results in a distortion of the parts and typically a concave shape on the stainless steel brazing mass and a convex shape to the outer cobalt-base alloy material. In summary, welding of the two dissimilar metals does not provide optimum coupling. Vacuum brazing is preferred but difficult to achieve.
What is needed, then, is an ultrasonic transducer assembly having a cobalt-base alloy housing, and a means to compensate for the distortion of the parts during the production process.